Hydrogeochemical modelling of origin, evolution and mechanisms controlling water resources quality in the Pra Basin (Ghana)

<p>Understanding the geochemistry of water resources is a prerequisite in the development of sustainable water resource management strategies. The Pra Basin is one of the few basins in Ghana with economic importance. The Basin is constituted by three river systems (Birim, Offin and Pra) and covers a total land size of approximately 2,300 km<sup>2</sup>. It traverses several towns and serves as the main water supply for communities and industry. Currently, the quality of water resources in the Pra Basin especially surfacewaters have been affected negatively as a result of activities such as illegal mining (e.g., the use of mercury for the extraction of gold), indiscriminate waste disposal, and poor farm management practices (e.g., inappropriate application of fertilizers and pesticides). Specific contaminants include mercury (Hg), arsenic (As), lead (Pb), iron (Fe), manganese (Mn), cadmium (Cd), selenium (Se), and nitrate (NO<sub>3</sub>). The Pra Basin is underlain by three rock formations, the Birimian Supergroup, the Tarkwain Formation and the granitoids. The mineral composition of the Birimian Supergroup comprises argillitic/pellitic sediment (plus or minus kerogen), sericite schist, and quartz-sericite schist. The granitoids comprise biotite (hornblende, muscovite), biotite gneiss, biotite schist, amphibolite partly of contact metamorphism, K-feldspar rich granitoid, two-mica or muscovite granite and monzonite, serecite schist, quartz-serecite, and garnet. The Tarkwaian rocks mineralogy also includes basaltic flow/subvolcanic rock and minor interbedded volcaniclastics, detrital sediment mainly sandstone and conglomerate ultramafic and minor mafic igneous rock. Samples of groundwater were collected from shallow (mainly hand-dug wells of depths < 10 m) and deep (mainly boreholes of depths >30 m) aquifers across the Pra Basin. Surfacewaters were collected from rivers and stream networks.  The samples were analysed for major ions, trace metals and stable isotopes (oxygen-18 and deuterium) using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), Ion Chromatography (IC), and Picarro L-2140i Ringdown Spectrometer at the GFZ laboratories. Multivariate statistical analysis and inverse geochemical modelling have been applied to around 100 water samples sourced from boreholes, hand-dug wells, and rivers of the Pra Basin to determine the chemical state of the waters. Specifically, the study seeks to (1) determine the origin and evolution of the geochemistry of both surfacewater and groundwater, (2) identify recharge and discharge areas, and (3) study sources and sinks of minerals including sulphates, carbonates, and silicates. The abundance of cations and anions are in the order of Na>Ca>K>Mg and HCO<sub>3</sub>>Cl>SO<sub>4</sub>>NO<sub>3</sub> (surfacewater), Na>Ca>Mg>K and HCO<sub>3</sub>>Cl>NO<sub>3</sub>>SO<sub>4</sub> (hand-dug well), and Na>Ca>Mg>K, and HCO<sub>3</sub>>Cl>NO<sub>3</sub>>SO<sub>4</sub> (boreholes). Our research findings demonstrate that geochemistry of water resources in the Pra Basin is mainly controlled by rock-water interaction. With the application of hydrogeochemical modelling, including silicate mineral weathering and ion exchange, significant processes controlling the basin’s hydrochemistry variations are quantified. The presented results will support the development of sustainable water resources management strategies and contribute to mitigating future contamination.</p>

Hydrogeochemistry and Related Processes Controlling the Formation of the Chemical Composition of Thermal Water in Podhale Trough, Poland

The most promising Polish region in terms of its geothermal resource potential is the Podhale Trough in the Inner West Carpathians, where the thermal water occurs in the Eocene-Mesozoic strata. The origin and conditions of formation of the chemical composition of the thermal water are different in a regional scale due to the impact of infiltrating water on the chemical compounds present in nearby thermal intakes, chemical processes responsible for the concentration of major elements and residence time. The article presents the regional conceptual model in regard to the factors controlling the chemistry of thermal water from Podhale Trough and the conditions of its exchange. It was allowed by performing the hydrogeochemical characteristics of studied water and analyzing its changes according to flow direction from HCO3-Ca-Mg type to SO4-Cl-Na-Ca and SO4-Ca-Mg types. The hydrogeochemical modelling was also made allowing identification of the impact of reservoir rocks on the formation of the chemical composition. For confirmation of the theories formulated and for more accurate interpretation of the results obtained from hydrogeochemical modelling, hydrochemical indices were calculated, i.e., rHCO3−/rCl−, rNa+/rCl−, rCa2+/rMg2+, rCa2+/(rCa2+ + rSO42−) and rNa+/(rNa+ + rCl−). The results revealed the most important processes evolving the chemistry of thermal water are progressive freshening of the thermal water reservoir, which in the past was filled with salty water, dissolution of gypsum, and ongoing dolomitization. Conducted research presents the important factors that in the case of increased exploitation of thermal water in the Podhale Trough, may influence the quality of thermal water in terms of its physical and chemical parameters.

Strategies to suppress internal loads of metals in sediments

Sediments may act as secondary sources of pollutants, but no known criteria establish such relationship. This study aims at evaluating internal loads of Ni from sediments due to changing redox conditions caused by application of water treatment such as hypolimnetic oxygenation. Through hydrogeochemical modelling and experimental evaluation, the distribution of Ni was evaluated. Sediment and water samples from a reservoir located in central Mexico were located in a reactor and Eh was controlled in ±50 mV steps between +300 and -450 mV. The release of Ni was evaluated in each step and the results were construed to the reservoir, considering that water-sediment interaction occurs in a 5-cm sediment layer with known density. Under oxidized conditions, the metal was accumulated in the sediment. With the dissolution of Fe at lower Eh, dissolved concentrations of Ni increased. Under even lower Eh conditions, these concentrations decreased again. This was related to the formation of iron sulphide. Combining experimental evaluation with hydrogeochemical modelling, allowed evaluating different scenarios of distribution of Ni. The knowledge of reductive dissolution of metals is therefore essential for decision-making to mitigate health effects as result of water treatment strategies.